Electrostatic printing process and developer composition therefor



June 16, 1959 E. c. GIAIMO, JR 2,890,968

PROCESS AND DEVELOPER COMPOSITION THEREFOR Filed June 2, 1955 W7IIIIIIIIIIIIIIIIIIIIII77I/z United States Patent ELECTROSTATIC PRINTING PROCESS AND DE- VELOPER COMPOSITION THEREFOR Edward C. Giaimo, Jr., Princeton, N.J., assignor to Corporation of America, a corporation of Delaware Application June 2, 1955, Serial No. 512,739

6 Claims. (Cl. 117-175) This invention relates to electrostatic printing and more particularly to improved methods and means for developing electrostatic charge patterns to make powder images.

An electrostatic printing process broadly is a process for producing a visible record reproduction or copy that includes as an intermediate step converting a light age or electrical signal to an electrostatic charge pattern on an electrically insulating surface. The process may also include the step of converting the charge pattern to a visible image which may be a substantially faithful reproduction of an original, except that it may differ in size and color.

A typical electrostatic printing process may include first producing a uniform electrostatic charge upon a photoconductive insulating surface. Typical photoccnductive materials useful in electrostatic printing are selenium, anthracene, and zinc oxide dispersed in an electrically insulating, film-forming vehicle. The electrostatically charged surface is exposed to a light image which selectively discharges different portions of the sur- I a 1 face. Those areas of the surface upon wlnch maxnnum light impinges lose relatively large proportions of their electrostatic charges, while those portions not exposed to light maintain substantially their full initially impressed charges. The light image impressed upon the charged photoconductive surface thereby forms an electrostatic image substantially corresponding to the light image. The electrostatic image is then developed by applying a developer powder which is held electrostatically to selected areas of the surface according to the electrostatic charge distribution. The developer powder thus forms a visible powder image which may be fixed directly to the photoconductive surface or, alternatively, transferred to another surface and fixed thereon.

A typical developing technique utilizes a magnetic brush comprising a developer mix including magnetic carrier particles and developer powder, called a toner, all maintained in a loose mass by a magnetic field. The magnetic field attracts and holds the magnetic carrier particles. The toner particles usually are non-magnetic. They are held in the brush by the triboelectric effect which imparts electric charges of mutually opposite polarity to the toner particles and the carrier particles. The triboelectric charges provide a physical attractive force to hold the "toner particles in the brush. When the magnetic brush comprising the loose mass of carrier and toner particles is swept across the surface bearing an electrostatic image, the triboelectrically charged toner particles brought into contact with the image-bearing surface are retained on selected areas of the surface by electrostatic attraction in substantial configuration with the electrostatic image.

One object of the instant invention is to provide developer mixes for use in a magnetic brush in an electrostatic printing process.

Another object is to provide an improved magnetic brush for electrostatic printing capable of selectively Patented June 16, 1959 printing either direct or reverse powder images of different respective colors.

Another object is to provide improved methods of electrostatic printing.

The foregoing objects and advantages may be accomplished in accordance with the instant invention which provides an improved magnetic brush and methods of using it such that a single magnetic brush may be utilized to form selectively either direct or reverse powder images. Additionally, the direct and the reverse images may be of different colors.

The invention provides a magnetic brush comprising magnetic particles of two different kinds which may differ in color and which have a relatively strong triboelectric effect with respect to each other. All of the particles of the brush are magnetic and are normally held in the brush by a magnetic field. The electrical potential of the brush is selectively varied with respect to the charged, image-bearing surface to be developed so that selectively one or the other of the two different kinds of particles in the brush may be deposited upon the charged surface to develop a visible powder image thereon. The variation of potential may be accomplished by applying a variable voltage from an external source between the brush and the charged-surface, by varying the polarity of the charge on the surface or both.

The invention will be described in greater detail in connection with the accompanying drawing of which:

Figure l is a schematic, cross-sectional, elevational view of apparatus for carrying out the process of the invention;

Figures 2 and 3 are schematic, cross-sectional, elevational and plan views, respectively, of apparatus for carrying out the process of the invention according to a second embodiment thereof; and,

Figure 4 is a greatly enlarged cross-sectional view of a toner particle according to the invention.

Similar reference characters are applied to similar elements throughout the drawing.

The developer powders, or toners utilized in the proccesses of the invention are composed of magnetic particles having insulating surfaces. The particles may be of ferrite materials such as magnetite or they may consist of any known magnetic materials such as iron coated with a colored material selected for its triboelectric properties.

In the practice of the invention it is relatively important that the two different toners of the magnetic brush have a relatively strong triboelectric effect with respect to each other. The triboelectric effect depends primarily upon surface properties. Thus, when the toners consist of coated magnetic particles, the cores of the particles may be identical and the triboelectric effect may be controlled by proper selection of the coating materials. The cores'alone may be magnetic so that the triboelectric effect may be controlled by selecting any desired coating material without regard to its magnetic properties.

A preferred magnetic brush may comprise approximately equal proportions of two magnetic powders prepared according to the following formulations:

Both the toners may be made up according to the following directions. The ingredients are reduced to powder form, preferably 200 mesh or finer and are mixed thoroughly. They are heated to about l-200 C.

to melt the resins and to form a completely fluid melt. The melt is stirred for approximately ten minutes while it is maintained at the specified temperature and is then allowed to cool and harden. The hardened pieces are broken up and ground in a mortar to reduce the material. to 200 mesh or smaller particles. The toners are then ready for use.

Piccolastic resin 4358A is an elastic, thermoplastic resin composed of polymers of styrene, substituted styrene and its homologs. Spirit Nigrosine Sill-3 is a black aniline dye having a color index of 864. Vinsol is a hard, brittle, dark-colored thermoplastic resin derived from pine wood and containing phenol aldehyde and ether groups. Carmine is a lake dye identified as No. 40 NF. (National Formulary).

The piccolastic resin composition is tribolectrically positive with respect to the Vinsol composition. When the two powders are mixed the Piccolastic powder, which is black, acquires a positive charge and the Vinsol powder, which is red, acquires a negative charge. The individual particles of both of the powders include magnetite and are, therefore, magnetic and may be magnetically held in a magnetic field to form a magnetic brush. If the brush is contacted to a negatively charged surface the positively charged black powder particles are attracted to the surface and tend to be deposited thereon. Conversely, the negatively charged red powder particles are repelled from a negatively charged surface and do not tend to be deposited thereon.

According to the invention the effect of the triboelectric charges acquired by the two powders is augmented by an additional electric potential as shown in Figure l.

A direct black powder image may be developed from a negatively charged electrostatic image according to one embodiment of the invention as shown in Figure l. A developer mix 23 composed of approximately equal weight proportions of the two toners heretofore described is held in a magnetic field to form a magnetic brush. The magnetic field may conveniently be provided by a permanently magnetized bar magnet 21 having a magnetic pole at one end 19 thereof. An electrical contact 25 is provided at any desired point along the bar magnet so that it may be maintained at a predetermined potential.

A battery 31 or other source of biasing voltage is con-- nccted to the magnet through a double-pole, double throw reversing switch 33 and a potentiometer 35. Thus, the applied biasing voltage may be changed in polarity and varied in magnitude as desired.

An electrically insulating layer 41 hearing a negatively charged electrostatic image is placed upon a grounded conducting backing plate 43. The switch and the potentiometer are adjusted to apply a positive voltage of about 700 volts with respect to the backing plate to the magnet 21 and the magnetic brush is then contacted across the surface of the layer 41. During the period of contact a unidirectional electric field appears between the magnetic brush and the backing plate due to the applied biasing voltage. This unidirectional field is in addition to and superimposed upon the electric field created by the electrostatic image. The positively charged black toner particles 67 from the magnetic brush are detached from the brush by the electric field and are deposited in areas of the layer 41 that have a negative electrostatic charge, thus producing a visible powder image thereon.

The visible powder image produced by the above described process has a relatively high contrast characteristic and a minimum amount of spurious toner deposit in the background areas. This set of characteristics is considered ideal for line drawings and line prints. By reducing the biasing voltage to decrease the unidirectional field, the contrast between the dark and light areas of the image may be reduced making it possible to obtain any desired contrast characteristic over a wide range of contrast values. When the biasing voltage is reduced to zero, the magnetic brush of the invention will still develop the negatively charged areas of the layer 41. Without a positive value of biasing voltage, however, a moderate amount of the black toner is frequently deposited in the background areas of the image to create spurious background darkening. Such spurious deposit may be reduced to a minimum by biasing the magnetic brush.

It the switch 33 is reversed and an increasing Voltage negative with respect to the backing plate is applied to the magnetic brush, a value is reached where the negatively charged areas of the layer 21 are no longer developed. Instead, the negatively charged red toner particles are deposited upon the background, or electrically discharged areas of the layer 21 to produce a reverse image. The contrast of the reverse image increases and the amount of spurious deposit in the charged areas of the layer decreases as the negative voltage is increased in magnitude. p I

A direct image formed of the red-colored Vinsol pow" der may be made according to another embodiment of the invention by using a positively charged electrostatic image. The procedure is identical with the procedure heretofore described in connection with Figure 1 for the production of a black powder direct image except that a positively charged electrostatic image is provided upon the surface of the layer 41 and the magnetic brush is biased negatively to about 700 volts.

According to this embodiment, the negatively charged red toner particles are deposited upon the positively charged surface areas of the layer 41 to produce a red direct image. The positively charged toner particles are held in the brush not only by the magnetic field but also by electrostatic forces. Image reversal and the contrast value and reduction of spurious deposits in background areas may be controlled by varying the polarity and magnitude of the bias voltage according to the same principles heretofore described in connection with the development of a negatively charged electrostatic image. In the case of the positively charged electrostatic image, however, the direct powder image is red and the reverse powder image is black.

A further embodiment of the invention comprises laying down a composite electrostatic image upon an insulating surface, i.e., an electrostatic image comprising separate areas of positive and negative charge, respectively. This image may be developed into a two-color powder image by a magnetic brush according to the invention without a biasing potential. The negative areas are developed by the triboelectrically positive toner and the positive areas are developed by the triboelectrically negative toner. In the case of the magnetic brush heretofore described the negative areas are developed with the black, Piccolastic toner and the positive areas are developed with the red, Vinsol toner. A relatively small bias on the magnetic brush may be needed in this embodiment to provide a particular, desired color balance.

Any positive or negative voltage may be used to bias the magnetic brush according to the invention so long as there is no electrical breakdown in the brush or in the layer 41 upon which the electrostatic image is developed. It is preferred, however, principally for purposes of convenience to limit the absolute magnitude of the voltage to about 1000 volts.

Many photoconductive materials exhibit rectifying characteristics and tend in varying degree to dissipate an electrostatic charge of one polarity. For example, pure, white zinc oxide can support a negative electrostatic charge for an extended period of time but dissipates a positive charge relatively rapidly. Its use is, therefore, preferably restricted to forming negatively charged electrostatic images. Selenium, on the other hand, appears to be polarizcd in the opposite direction and is better able to support a positive charge than a negative one.

One photoconductive material capable of satisfactorily holding both negative and positive electrostatic charges is a pink form of zinc oxide manufactured by a method described in the US. application of Conn et a1., Serial No. 329,473, filed January 2, 1953, and assigned to Merck and Co., Inc. This method comprisesheating ammoniated zinc carbonate at a temperature-of 100- 400 C. Preferably, the carbonate is heated at 250 C. for about 1-2 hours or until all of the ammonia and carbon dioxide are released. At this point a weight loss of about 43% will be realized and the resulting product will have a particle size of about 100-200 A. The ammoniated zinc carbonate may be first prepared by adding white Zinc oxide to a solution of ammonium carbonate in 28% ammonia water, blowing in carbon dioxide until no further separation of solid occurs, filtering out the solid, which is the ammoniated zinc carbonate, washing and drying.

in this application the terms reverse and direct when used to refer to powder images correspond generally with the terms negative and positive, respectively, as ordinarily used in respect of common silver halide photographic processes. The terms may best be explained by reference to an example in which an electrostatic charge image is produced upon a charged surface by projecting light upon the surface through a transparency. A direct image developed upon the surface has dark areas corresponding to the dark portions of the transparency and light areas corresponding to the light portions of the transparency. Conversely, a reverse image has light areas corresponding to the dark portions of the transparency and dark areas corresponding to the light portions of the transparency.

The developer powders or toners of the developer mix of the invention may be chosen from a large class of materials. It is essential in the practice of the invention that the individual particles of the toners be magnetic and that the two toners comprising the magnetic brush have mutually different triboelectric properties so that one toner will become positively charged and the other negatively charged when they are brought into contact with each other.

The magnetic properties of the toners may be provided by utilizing any magnetic material such as iron, nickel, magnetite or a ferrite either as the complete toner or as core material for the individual particles of a toner. A relatively high degree of latitude in the choice of materials and a wide range of color variations may be readily secured by use of a coated core type toner such as the particle shown in Figure 4, and this type is, therefore, the preferred type according to the invention.

In making a coated core toner the magnetic particles are preferably first reduced in size to at least 200 mesh or smaller. The particles may be coated by fusing a selected coating material upon them as heretofore described or by dissolving the coating material in a solvent, mixing the core particles into the solvent and evaporating the solvent. Both these methods appear to provide equally satisfactory results. The fusing method has the advantage of greater ease in processing, but is diflicult to use with relatively high melting point coating materials and is obviously unsuitable for use with thermosetting resinous coating materials which decompose instead of melting upon heating.

The selection of coating materials for the toners according to the invention is not critical except for the requirement that they have mutually different triboelectrio properties, i.e., they must have different positions in the triboelectric series so that they will charge each other with mutually opposite electrostatic charges by means of surface contact. The two coating materials cannot be identical in composition.

Most materials are triboeleotrically effective with most other materials but there is no known rule by which their triboelectric effects can be predicted. Many different triboelectric series have been empirocally determined.

These series comprise lists of difierent materials usually arranged in progressive order of positive triboelectric properties such that each material in the series is charged positively upon surface contact with any of the materials below it in the series. See, for example, The Tribeelectric Series, Table 482, the Smithsonian Physical Tables; 8th 'Revised Ed., page 408. See also av table by Lehrnicki in The American Dyestuffs Reporter, 38, page 853, 1949.

The position of a material in the triboelectric series may be greatly changed by admixing other materials to it. For example, in the coating compositions heretofore described the admixture of carbon black and the dye Nigrosine SSB to the Piccolastic resin increasesthe positive ,triboelectric property of the resin. Also, the use of carmine dye mixed with Vinsol, the phenolic type resin increases Vjnsols negative triboelectric effect. In the case of Piccolastic resin a further addition of a small proportion of a dye sold under the trade name Iosol Black (National Aniline Division, Allied Chemical and Dye Corporation) makes the resin triboelectrically even more positive.

In the development of a reverse or direct image according to the instant invention the magnetic field of the magnetic brush and the biasing voltage applied to it cooperate to hold one of the toners in the brush, thereby to reduce spurious deposits of the unwanted toner. The unwanted toner is held in the brush and acts as a carrier to perform essentially the same functions as carrier materials in previous magnetic brushes.

The biasing voltage cooperates with the electric field of the electrostatic image to overcome the attraction of the magnetic brush so that the desired toner particles are released from the magnetic brush and deposited upon the electrostatic image to-develop it into a powder image. Depending uponthe polarities of the biasing voltage and the electrostatic image, the development may be either direct or reverse. When the brush bias is of the opposite polarity from the electrostatic image, with respect to a common reference point, adirect powder image is developed. When the brush bias is of the same polarity and suificient in magnitude to overcome the other electrostatic and magnetic forces a reverse image is developed.

. By utilizing toners of different respective colors such as those heretofore described, and by controlling the polari ties of the brush bias and the electrostatic charge image,

both direct and reverse powder images may be developed by either of the two colored toners of the magnetic brush.

The theory underlying the reversal processes of the invention is not completely understood. It is believed that a virtual ground plane is automatically and inherently provided by the magnetic brush immediately adjacent to the electrostatic image. The extreme closeness of the virtual ground plane to the electrostatic image greatly enhances the image field effect. By superimposin an electric field of the desired polarity upon the image field, the image field is effectively enhanced or suppressed to produce a wide range of direct or reverse contrast values in the developed visible image. The effect may be due to contact charge of the developer powder particles at the brus'h ends or to an electric field additional to the electrostatic image field or both.

Referring now to Figure 2, the improved methods and means of the invention may be incorporated in a continuous electrostatic printing process and apparatus. In this apparatus a continuous web comprising a paper substrate 51 having on one surface thereof a photoconductive insulating coating 53 comprising a powdered photoconductor such as zinc oxide dispersed in an electrically insulating film-forming vehicle such as polyvinyl acetate is unwound from a roll 55. The web first passes a station where a uniform electrostatic charge of controllable polarity and magnitude is impressed on the photoconductive coating 53 by a corona, discharge from a series of wires 57 connected to a voltage source 59 through a double-pole, double-throw reversing switch 61 and a potentiometer 63. Alternatively, the surface may be charged by rubbing or by simple contact with a biased conductive roller.

The web next passes to a station where an electromagnetic radiation image is directed upon the photoconductive surface, illustratively by projection of light through a photographic transparency by means of a projector 65. The electromagnetic radiation image discharges selected portions of the charged surface to produce thereon an electrostatic image substantially corresponding to the projected electromagnetic radiation image.

At the next station the electrostatic image on the surface of the photoconductive coating 53 is developed into a direct powder image by the method of the invention. A grounded rotary pole piece 20 of a magnetic structure isprovided with spaced, parallel, inclined, elliptical discs 22 facing the web. A magnetic field is maintained between a fixed magnetic pole piece 45 spaced from the elliptical discs 22 and on the opposite side of the web from the discs through magnetic pieces 47 and 49 and through a gap therebetween. A reservoir 71 holds a quantity of developer mix in contact with the discs 22. A shield 44 located between the fixed pole piece and the web is maintained in contact with or closely spaced behind the paper substrate 51 and is connected to a voltage source 31 through a double-pole, double-throw reversing switch 33 and a potentiometer 35. The switch 33 and the potentiometer 35 are adjusted to provide the desired biasing voltage upon the shield 44.

As in the previous example the biasing voltage may be of about 700 volts magnitude. To produce a direct image as in the instant example, the shield is made negative with respect to ground so that the grounded magnetic brush contacted to the web by the rotating discs 22 is at a positive potential with respect to the shield. As the rotary pole piece 20 rotates in a clockwise direction, developer mix forms on the periphery of the discs 22 in brush-like filaments 23 and is carried upwardly and swept across the surface of the photoconductive coating 53 passing the station. The black, positively charged toner particles 67 are deposited upon negatively charged areas of the coating to produce a direct visible powder image on the photoconductive coating in substantial configuration with the electrostatic image.

Direct or reverse visible powder images of black or red color may be produced as desired by changing the bias applied to the shield and the polarity of the corona discharge according to the principles heretofore de scribed. The background deposits and the contrast value of the powder images may be varied by adjusting the magnitude of the biasing voltage on the shield 44 as previously described in connection with the embodiments illustrated in Figure l.

The continuous web bearing the visible powder image held thereon by electrostatic charges now passes to a station where the visible image is fixed to the photoconductive coating 53. For this purpose a radiant heater comprising a resistance wire 73 connected to an electric current source 75 through an adjusting potentiometer 77 is maintained in closely spaced relationship with the web. Heat radiated from the wire 73 softens the thermoplastic resin of the toner causing it to adhere to the photoconductive coating 53.

After the powder image is fixed the continuous web may be wound on the take-up roll 56, or it may be cut into sheets and stacked or utilized directly for any desired purpose.

The visible powder image may, alternatively, be fixed to the photoconductive layer by other means such as, for example, by spraying with an adhesive or by coating with a material capable of softening either the photoconductive coating or the toner. Such other means may be desirable particularly when the toner comprises thermosetting instead of thermoplastic resins.

The visible powder image may also be transferred to another surface and fixed thereon by any convenient means. If this is done, the web and the photoconductive coating 53 may be returned through the cycle and reused.

There have thus been described improved magnetic brushes for use in developing electrostatic charge images and methods of using them to develop such images into visible powder images.

What is claimed is:

1. A developer mix for developing a latent electrostatic image, said mix consisting essentially of a dry mixture of loose movable particles of two separate magnetic powders of substantially equal average particle size and weight; the particles of the first of said powders having a triboelectric relationship of opposite polarity with respect to the particles of the second of said powders, the particles of said first powder thereby being electrostatically-charged in a first polarity through triboelectric action by contact with the particles of said second powder and being attractable by charged areas of a second polarity of said latent electrostatic image when moved thereacross and also being adapted to attract particles of said second powder and to remove them from areas of said first polarity of said latent electrostatic image when moved thereacross; and the particles of said second powder being correspondingly electrostatically charged in said second polarity through triboelectric action by contact with particles of said first powder and being attractable by charged areas of said first polarity of said latent electrostatic image when moved thereacross and also being adapted to attract particles of said first powder and to remove them from areas of said second polarity of said latent electrostatic image when moved thereacross; the particles of both of said powders being of a size such that the gravitational force on each granule is less than the magnetic and electrostatic forces thereon.

2. An electrostatic printing developer mix comprising two magnetic powders, the first of said powders consisting essentially of magnetic cores coated with a first insulating material having a triboelectric property of one polarity, the second of said powders consisting essen tially of magnetic cores coated with a second insulating material having a triboelectric property of opposite polarity to that of said first insulating material, the coated particles of said second powder being substantially equal in average particle size and weight to the particles of said first powder.

3. An electrostatic printing developer mix comprising two magnetic powders, the first of said powders consisting essentially of magnetic cores coated with a first insulating material of one color and having a triboelectric property of one polarity, the second of said powders consisting essentially of magnetic cores coated with a second insulating material of a difierent color and having a triboelectric property of opposite polarity to that of said first insulating material, the coated particles of said second powder being substantially equal in average particle size and weight to the particles of said first powder.

4. The electrostatic printing developed mix of claim 3 wherein said first and said second insulating material comprise pigmented resins.

5. Method of controllably developing a visible powder image in at least one of two preselected colors upon a surface bearing an electrostatic charge image, said charge image comprising localized positive charges and localized negative charges upon difierent areas of said surface, the polarity of said charges being taken with respect to a point of reference potential, said method comprising contacting a developer mix across said surface and varying the polarity and the magnitude of the potential of said developer mix with respect to said point of reference potential, said developer mix comprising two different magassases netic powders, the particles of said powders consisting essentially of magnetic cores coated with insulating materials, the insulating material of one of said powders having a triboelectric relationship of opposite polarity with respect to the insulating material of the other of said powders held in a loose mass by a magnetic field, thereby controllably to deposit portions of the different ones of said powders upon different areas of said surface in accordance with the charge image thereon.

6. In an electrostatic printing process employing an electrostatic image bearing surface, said image comprising areas of positive charge and areas of negative charge with respect to a reference potential, the method of corn trollably developing a visible powder image in areas of at least one of said charges comprising: magnetically transporting a developer mix across said surface and varying the polarity and magnitude of the potential of said developer mix with respect to said reference potential, said developer mix comprising two magnetic powders consisting essentially of magnetic cores coated with insulating material, the insulating material of one of said powders having a triboelectric relationship of m opposite polarity with respect to the insulating material of the other of said powders, thereby to controllahly deposit at least one of said developer powders upon said surface in accordance with the electrostatic image there-- Refierenees (Cited in the file of this patent UNET PATENTS 

5. METHOD OF CONTROLLABLY DEVELOPING A VISIBLE POWDER IMAGE IN AT LEAST ONE OF TWO PRESELECTED COLORS UPON A SURFACE BEARING AN ELECTROSTATIC CHARGE IMAGE, SAID CHARGE IMAGE COMPRISING LOCALIZED POSITIVE CHARGES AND LOCALIZED NEGATIVE CHARGES UPON DIFFERENT AREAS OF SAID SURFACE, THE POLARITY OF SAID CHARGES BEING TAKEN WITH RESPECT TO A POINT OF REFERENCE POTENTIAL, SAID METHOD COMPRISING CONTACTING A DEVELOPER MIX ACROSS SAIDSURFACE AND VARYING THE POLARITY AND THE MAGNITUDE OF THE POTENTIAL OF SAID DEVELOPER MIX WITH RESPECT TO SAID POINT OF REFERENCE POTENTIAL, SAID DEVELOPER MIX COMPRISING TWO DIFFERENT MAGNETIC POWDERS, THE PARTICLES OF SAID POWDERS CONSISTING ESSENTIALLY OF MAGNETIC CORES COATED WITH INSULATING MATETIALS, THE INSULATING MATERIAL OF ONE OF SAID POWDERS HAVING A TRIBOELECTRIC RELATIONSHIP OF OPPOSITE POLARITY WITH RESPECT TO THE INSULATING MATERIAL OF THE OTHER OF SAID POWDERS HELD IN A LOOSE MASS BY A MAGNETIC FIELD, THEREBY CONTROLLABLY TO DEPOSIT PORTIONS OF THE DIFFERENT ONES OF SAID POWDERS UPON DIFFERENT AREAS OF SAID SURFACE IN ACCORDANCE WITH THE CHARGE IMAGE THEREON. 